BTW, just on the subject of ultrasonics, I came across
this the other day whilst Googling

"Ultrasound beyond the speed of light"

http://www.acoustics.org/press/150th/Mobley.html

"So it may be hard to believe that by sprinkling in some small plastic
beads, water can be made to support ultrasonic pulses with speeds
faster than light. Not only can ultrasound pulses outrun light in this
watery mixture, they can also propagate in negative time, apparently
reaching a more distant point before a closer one. How is this possible?
What does it mean? How does it fit within the known laws of physics?"

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